1. Field of the Invention
The invention relates to a system for the generation of an estimated time variant signal for suppression of a primary signal, comprising:
means for generating at least one reference signal; PA1 a control unit at least provided with a digital filter, a first input for receiving the reference signal, a second input for receiving an update signal for updating coefficients of the digital filter and an output for providing a cancellation control signal; PA1 cancellation-generating means which are connected to the output of the control unit for the generation of a cancellation signal, which is intended, after propagation along a secondary transfer path having a secondary path transfer function, to be added as the estimated time variant signal at an addition point to the primary signal in order to provide a residual signal, PA1 sensor means for measuring the residual signal at the addition point and for providing an error signal; PA1 update means provided with an input which is connected to the sensor means and an output for providing the update signal. PA1 the system also comprises an adaptation unit which has an input for receiving the at least one reference signal and an output for providing a filtered-x signal, which output is connected to a further input of the update means; PA1 the adaptation unit has a transfer function which is the same as the transfer function of the secondary path; PA1 means are provided for updating the filter coefficients with the aid of the so-called "projection algorithm" and PA1 the means for generating the reference signal generate the reference signal in such way that an input correlation matrix defined as follows ##EQU1## has an eigenvalue distribution which has a value which is substantially equal to 1 after as few steps as possible. PA1 .theta..sub.c,v (k) is an L-dimensional filter coefficient vector when L cancellation signal generators are present; PA1 .gamma.=step size parameter; PA1 R.sub.v (k) is an LxM matrix which contains input signals r.sub.v,l,m (k) as elements when M sensors are present; PA1 .epsilon.(k)=error signal vector with M elements originating from the M sensors. PA1 the means for generating the reference signal are equipped to generate the following reference signal: EQU .phi..sub.f (k)=[.phi..sub.1 (k).phi..sub.2 (k) . . . .phi..sub.I (k)].sup.T PA1 where: EQU .phi..sub.v (k)=exp(j2.pi.f.sub.v k.DELTA.t) PA1 the update unit is provided with means for calculating the update signal as follows: ##EQU2## where: .gamma.=step size PA1 the control unit is provided with means for calculating the filter coefficients as follows: EQU .theta..sub.c,v (k)=.theta..sub.c,v (k-1)-up.sub.v (k) PA1 at least one loudspeaker, at least one vibration generator, at least one adjustable flow-induced noise source or a combination thereof, and the sensor means can be selected from the following means:
2. Discussion of the Related Art
A system of this type is disclosed in U.S. Pat. No. 4,878,188.
The known system can be used, inter alia, to cancel out irritant noise (the primary signal) from a rotary machine and comprises means for generating a reference signal, which means, for example, comprise a tachometer of a rotary machine or an optical sensor for measuring the speed of revolution of the rotary machine. In addition, the known system comprises a signal processor for generating a cancellation control signal, which is supplied to cancellation signal-generating means for generating a cancellation signal, which as far as possible has to cancel out the primary signal within a specific area. The residual signal in that area is measured using suitable sensor means, the output signal from which is fed to update means. The latter means produce an update signal for the signal-processing means in order to update the filter coefficients of an adaptive filter present therein. The update signal is derived form the residual signal and can, for example, be determined with the aid of the least mean squares (LMS) method and is aimed at causing the residual signal to become as small as possible.
In the system according to the US Patent, in the case of a multi-channel embodiment, the reference signal is supplied via n base reference signals on n parallel branches to the signal-processing means. Each base reference signal is fed to a cosine generator and a sine generator. The cosine generator produces a base cosine signal with a specific base frequency and a number of higher harmonics thereof. The sine generator produces a base sine signal with a specific base frequency and a number of higher harmonics thereof. All cosine and sine signals are multiplied by suitable filter coefficients of the adaptive filter. Phase means are used to ensure that the adaptation of the filter coefficients takes place in such a way that the filter characteristics remain within 90.degree. of the primary signal. A summation unit finally adds together all cosine and sine signals modified in this way in order to generate the cancellation control signal.
The convergence speed of the algorithms used in the prior art is found to be moderate for various types of primary signals, for example for rapidly changing periodic signals which, for example, arise in the exhaust of a car, and for pulse-like signals. In this context the convergence speed is defined as the speed with which a control system used proceeds to the state of as complete as possible noise suppression. The convergence speed of the known algorithms is found not to be optimum in multi-channel systems as well.